Communicating Benefits - early and often
As your event day nears, make sure all avenues of communication to your attendees reference downloading the app ahead of time. This should include your website, social media pages, and email blasts.
Make sure attendees know how to connect to Wi-Fi at the event. Have plenty of signage and ensure staff is equipped to answer questions about connecting.
Have a few charging stations set up throughout the venue to eliminate "single-wall-socket-huddle". The most common types of cables to have available are Apple's Thunderbolt Cable, Apple's 30-pin cable, and Micro USB (the standard for Android phones). Having these 3 types of cables readily available will cover 99% of mobile devices out in the market.
Lend A Helping Hand
Station a Mobile App Help Desk near the registration area. Staff should have the basic knowledge necessary to help users download, log in, and reset their password (if necessary).
Making information available mobile-only will help drive adoption of the app and has the added benefit of saving you money! Try and eliminate as much paper from your event as possible, and where you do need to use printed materials make the most of it by also including links or QR codes to make it easy for attendees to find the app.
Making the most of the gamification options within the app will encourage attendee's use of the app and keep your event fun and exciting. If possible, link points or achievements in the app with live prizes at the event.